Bacteria constantly produce molecules called acylated homoserine lactones (AHLs) and release them into the environment.
(The LuxR proteins comes into play later.)

If other bacteria are not in the vicinity, the AHLs will soon degrade and bacterial “silence” will prevail.

But if enough AHL-producing bacteria are nearby, the concentration of AHLs outside the bacteria will rise, signaling the increase in cell density around them.

(1) When a quorum (or critical number) of bacterial cells is reached,
AHLs are able to bind to receptor proteins called LuxR. (2) That changes
LuxR’s shape so that it can bind to DNA in the bacteria, like a key in a
lock.

The LuxR “key” turns on a string of genes (1) that produce the components (2) to make an enzyme called luciferase (3), which generates light (bioluminescence). By similar biochemical processes, bacteria can produce several other collective behaviors.

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